ABSTRACT: Infectious salmon anaemia (ISA) is a serious disease responsible for high morbidity in farmed Atlantic salmon Salmosalar in Norway, Scotland and New Brunswick, Canada. Recent attempts to identify different strains of ISA virus
(ISAV) based on nucleotide sequence variation have shown that the Norwegian and Scottish samples are similar to one another but markedly different from New Brunswick samples. These data may suggest the presence of different strains on each side of the
Atlantic but no functional difference has been found with either strain. We describe the first identification and characterisation of ISAV in Atlantic salmon from Nova Scotia, Canada. Further, salmon infected with the Nova Scotia ISAV do not show typical
ISAV pathology or mortality. Sequencing of this new strain showed it to possess greater similarity to ISAV from Norway and Scotland than to ISAV from New Brunswick. These findings are discussed in terms of a possible origin of the Nova Scotia ISAV strain
and the existence of an avirulent ISAV strain. The impact of current strain variation studies on our knowledge of ISAV is also discussed.